The general goal of this proposal is to determine the lineage pathways through which human primitive precursor cells develop intrathymically into functional T cells. A major problem in understanding human T cell development is the lack of knowledge of the intrathymic events leading to T cell receptor expression. The specific questions studied in this research include the lineage relationships among early T cell populations prior to membrane T cell receptor expression and NK-like cells. The definition of cytokine and cellular, especially thymic epithelial cell and thymic macrophage, interactions involved in T lineage and NK lineage determination are a principal focus of the planned research. Specific aims include 1) to analyze putative precursor populations for surface phenotype, cell cycle status, and cytokine receptor expression and to generate in vitro clones derived from these precursors; 2) to analyze the T cell receptor (TCR) DNA, MRNA, and protein expression of intrathymic progenitor populations and of thymocyte clones derived from progenitor T cell populations; 3) to determine whether intrathymic progenitor populations contain a precursor to NK cells or, rather, T cell progenitors "mature" into NK cells under in vitro culture conditions using analysis of CD3 and TCR gene and protein expression; and 4) to determine the functional capabilities of thymocyte clones derived from precursor populations under different culture conditions. Thus, this proposal will define intrathymic precursor populations and initial maturation pathways. These studies should assist the understanding of normal T cell maturation and abnormal T cell maturation such as occurs in immunodeficiency diseases and is acquired with HIV infection. By determining T cell precursors and their differentiation capability, the possibility of transplantation of these precursors into patients with immunodeficiency states may be enhanced.